Abstract
This paper describes investigations of a 10,000 m3 (12,000 cuyd) debris flow that mobilized from coarse-grained Holocene stream terrace deposits near Cordova, New Mexico. Field evidence suggests that mobilization occurred very near the main scarp—which intersects a steeply dipping, clayey paleosol directly beneath the terrace—and that the debris was flowing before it reached the nearby Rio Quemado. Recompacted stream terrace samples were contractive in two out of three direct shear tests, and we calculated the approximate mobility index of the terrace sediments to be greater than unity. If they were completely saturated, therefore, the terrace sediments could have been mobilized from a small debris slump without the addition of any water. In order to investigate this possibility, we performed a series of limit equilibrium stability analyses and found that a phreatic surface coincident with the ground surface would have been necessary in order for sliding to occur. We conclude that the failure was probably due to the unprecedented development of a perched water table above the paleosol, which caused sliding and subsequent mobilization. The two most likely sources of water were seepage from an irrigation ditch above the failed slope and over-irrigation of the terrace, which was being used as a pasture. The presence of weakly developed calcic soil horizons suggests that terraces along the Rio Quemado have been well-drained and stable throughout most of their history, which is confirmed by regional landslide inventory maps. Therefore, we recommend that geologic inventories be supplemented by quantitative mechanical analyses in order to properly assess the potential for slope instability due to unprecedented activities such as irrigation, construction, or logging.
